The present invention generally relates to a backup system for a utility power grid and, more particularly, is concerned with an uninterruptible power supply (UPS) system and method having a fast-start capability and a low-energy storage requirement.
Present day computer data centers and internet service providers require highly reliable sources of uninterruptible power. Dual fed utility grid systems plus uninterruptible power supplies with back-up generation are utilized to meet the stringent demands for critical power. A conventional UPS system normally includes a DC-AC inverter, transfer switches, an energy storage unit, oftentimes using massive lead acid batteries, and, in many cases, an alternator driven by a heat engine. During a fault in the dual-fed grid source, DC power from the energy storage unit (usually batteries) is converted to the proper AC voltage, current, and frequency and connected to the critical loads via the transfer switches. Further, many of the conventional UPS systems, rated to provide critical power for long periods of time, have a heat engine and an alternator in addition to the battery energy storage unit. In these conventional UPS systems, the battery energy storage unit is required to provide the electrical power to the critical load until the heat engine is cranked and started and the alternator has achieved a stabilized output.
A problem exists, however, in that conventional engine driven alternator UPS systems with 100 kW-1 MW power ratings typically require at least 15 seconds to start the heat engine and alternator and stabilize the electrical output prior to connecting the alternator to the critical load via the transfer switches. During the initial 15 seconds, the energy storage unit supplies all of the energy to the critical load via the inverter. Therefore, the energy storage unit is required to supply rated power for at least 15 seconds, meaning that for a 1 MW rated UPS system at least 4.2 kWh of energy storage is required.
Consequently, a need exists for an innovation which will greatly reduce the duration of rated power required to be supplied by the energy storage unit and thus eliminate the need to use massive lead acid batteries in the energy storage unit of the UPS system.
The present invention provides an uninterruptible power supply (UPS) system and method designed to satisfy the aforementioned need. The UPS system of the present invention has a fast-start capability that greatly reduces the duration of rated power required to be supplied by an energy storage unit and thus eliminates the need to use massive lead acid batteries in the energy storage unit of the UPS system.
In one embodiment of the present invention, an uninterruptible power supply system having a standby mode of operation and a backup mode of operation is provided which includes a heat engine having a crankshaft, an alternator having an input shaft and an output electrically connected to an electrical power grid, a flywheel disposed between the heat engine crankshaft and alternator input shaft, a first motion transmitting mechanism disposed between the flywheel and heat engine crankshaft and actuatable between engaged and disengaged conditions for correspondingly drivingly coupling and decoupling the flywheel to and from the heat engine crankshaft, a second motion transmitting mechanism providing a one-way drive coupling between the flywheel and alternator input shaft, a third motion transmitting mechanism disposed between the flywheel and alternator input shaft and actuatable between engaged and disengaged conditions for correspondingly drivingly coupling and decoupling the flywheel to and from the alternator input shaft, a source of AC electrical energy connected to the electrical power grid, and an uninterruptible power supply system control connected to the electrical power grid and to the heat engine and first and third motion transmitting mechanisms for controlling operation of the heat engine and first and third motion transmitting mechanism so as to change the system from a standby mode of operation to a backup mode of operation in response to detecting a fault in the power grid. The second motion transmitting mechanism provides the one-way drive coupling such that in a first direction (a first polarity of torque) from the alternator to flywheel the second motion transmitting mechanism operates in an overrun mode in which the alternator input shaft can rotate at a speed faster than that of the flywheel whereas in a second direction (a second polarity of torque), opposite the first direction, from the flywheel to alternator the second motion transmitting mechanism is operates in a locked mode in which the flywheel can rotate up to the same speed as that of the alternator input shaft. When the third motion transmitting mechanism is in the disengaged condition the second motion transmitting mechanism permits the alternator input shaft to overrun the flywheel such that no driving motion is transmitted from the alternator input shaft to the flywheel whereas when the third motion transmitting mechanism is in the engaged condition drive torque can be transmitted in either direction between the flywheel and alternator input shaft so that the alternator input shaft and flywheel can rotate at the same speed.
In another exemplary embodiment of the present invention, a method of operating an uninterruptible power supply system is provided comprising the steps of operating the system in a standby mode by maintaining the system electrically connected to a power grid in a state of readiness to change to a backup mode in response to detecting a fault in the power grid, and operating the system in the backup mode by supplying electrical power to the power grid sufficient to overcome the power loss caused by the detected fault and to rotate a heat engine of the system up to a speed where the heat engine can be started and accelerated to full operation by introduction of a fuel energy source independent of the power grid.